A considerable number of tannic acids occur in nature. Chemically, these acids are described as polymers of different hydroxybenzoic acids. Generally, when the term tannic acid is employed, as in the present case, the acid referred to is gallotannic acid, the internal ester of gallic acid also frequently referred to as tannin.
Tannic Acid consists of an amorphous powder, glistening scales or spongy masses varying in color from yellowish-white to light brown. Tannic acid is very soluble in water, glycerine or alcohol.
Tannic acids are usually obtained from glycosides which consist of several molecules of a tannic acid in combination with glucose.
Commercially available, tannic acid, also known as Tannin, have a complex and non-uniform chemistry usually contain from about 5% to 10% by weight water, has a molecular weight of about 1700 and is typically produced from Turkish or Chinese nutgall.
Phenylephrine, known chemically as L-m-hydroxy .alpha. [(methylamino)methyl] benzal alcohol, is a synthetic, optically active sympathomimetic amine which has one hydroxyl group on the benzene ring. The hydroxyl group is placed in the position meta to the aliphatic side chain. The meta position affords optimal activity and phenylephrine (neo-synephrine) replaced an older preparation, synephrine, in which the hydroxyl was in the para position.
Phenylephrine hydrochloride is available in the form of the levoratory isomer, a white, odorless, non-hygroscopic, crystalline compound possessing a bitter taste. Phenylephrine chloride has a melting point of 140-145.degree. C. and is freely soluble in water and alcohol.
Pyrilamine is one of the oldest and most enduring antihistaminic drugs, known chemically as N-[(4-methoxyphenyl)methyl]-N',N'-dimethyl-N-2-pyridinyl-1,2-ethanediamine , its preparation is disclosed in U.S. Pat. No. 2,502,151 and is an oily liquid. Pyrilamines hydrochloride salt is very soluble in water and has a melting point of 143-143.5.degree. C. whereas the maleate salt is slightly soluble in water, benzene and ether and has a melting point of 100-101.degree.C.
Antihistamine and decongestant compounds in the form of their free bases as well as their salts, e.g. hydrochloride, citrate, maleate, tannate, etc., are well known. Antihistamines and decongestants in the form of their tannate salts are very desirable because such salts are generally stable and may be combined in such form without any untoward side effects.
Antihistaminics in the form of their tannate salts are typically prepared by reacting the antihistamine/decongestant free bases, e.g. phenylephrine and pyrilamine, with tannic acid in the presence of a volatile solvent, usually isopropanol. Typically, in the conventional isopropanol route, the antihistaminic/decongestant free bases and the tannic acid will be present in the isopropanol at a concentration of about 20% based on the weight of the reaction mixture. The reaction mixture is stirred for about one hour while maintaining the mixture at 60-70.degree. C. The reaction mixture is cooled to room temperature and then filtered, washed with isopropanol and then vacuum dried. Alternative routes to the tannate salts are described in U.S. Pat. No. 5,599,846 and U.S. Pat. No. 5,663,415.